1. Field of the Invention
The present invention relates to an engine for a motorcycle having an improved arrangement of a rotating shaft, a clutch unit, gears, and other units in an engine case.
2. Related Art
Conventional examples are disclosed in Japanese Unexamined Patent Application Publication No. 4-282054 (patent publication 1) and No. 2005-016511 (patent publication 2).
An engine described in the patent publication 1 includes a transmission input shaft (counter shaft) and a transmission output shaft (drive shaft) which are disposed in parallel to each other and rotatably supported behind a crankshaft. The crankshaft is rotatably supported in a front portion in an engine case and extends laterally. The engine also includes a clutch unit disposed on a right end of the transmission input shaft. A small-diameter primary drive gear disposed on the crankshaft meshes with a large-diameter primary driven gear disposed on the clutch unit. The rotation of the crankshaft is transmitted to the clutch unit at reduced speed and, via the clutch unit, transmitted to the transmission input shaft and the transmission output shaft.
The rotation of the transmission output shaft is transmitted to a longitudinally extending final output shaft via a bevel gear mechanism and is then transmitted to a rear wheel by means of drive shaft connected to the rear portion of the final output shaft. The bevel gear mechanism and the final output shaft are incorporated in a bevel gear housing disposed on a left side of the engine case.
The transmission input shaft has a hollow structure in which a clutch release rod is inserted. A clutch release mechanism mounted on an extension of a left end of the transmission input shaft serves to connect and disconnect the clutch unit through the clutch release rod. The clutch release mechanism is mounted on the bevel gear housing. The clutch release rod passes through the bevel gear housing.
An engine described in patent publication 2 has a structure in which a balancer shaft, a transmission input shaft, and a transmission output shaft are rotatably supported in parallel behind a crankshaft. The rotation of the crankshaft is transmitted to the balance shaft at the same speed, the rotation of the balancer shaft is transmitted to a clutch unit disposed on the transmission input shaft at a reduced speed, and the rotation of the clutch unit is transmitted to the transmission input shaft and the transmission output shaft.
However, for the engine described in patent publication 1, since the rotation of the crankshaft is transmitted to the clutch unit at the reduced speed, a shaft torque acting on the clutch unit becomes large. This results in the increasing of the capacity (dimensions) of the clutch unit, thus causing a problem in which the clutch actuation force is inevitably large and the size of the engine case is increased.
In addition, since the clutch unit is disposed on the right end of the transmission input shaft in this engine, both the primary driven gear disposed on the clutch unit and the primary drive gear meshing with the primary driven gear are situated in the rightmost side in the engine case, and therefore, only a clutch cover on the right side of the engine case is present between both gears and the outside thereof. As a result, noise caused by the engagement of the primary driven gear and the primary drive gear is propagated to the outside, and quietness of the engine is hence degraded.
Moreover, since the clutch unit is mounted on one end of the transmission input shaft, the clutch release mechanism on the other end of the transmission input shaft interferes with the bevel gear housing positioned behind the clutch release mechanism. Therefore, the clutch release mechanism is required to be set outside the bevel gear housing. This causes a problem in which the size of the engine case is increased laterally.
For the engine described in patent publication 2, although the rotation of the crankshaft is transmitted to the balancer shaft at the same speed, the rotation of the balancer shaft is transmitted to the clutch unit at the reduced speed. Therefore, the rotation torque acting on the clutch unit becomes large, which requires an increase in the capacity of the clutch unit. The clutch actuation force and the size of the engine case are inevitably increased.